[ODE] Vehicle Speed (Generic Units to Real World Units)

[Jeff Weber]

I understand units in ODE are generic and i can choose whatever units I want 
as long as I am consistent.

I currently have things set up so that 8 ode units = 1 meter.  This is 
arbitrary and was chosen for reasons related to the graphics engine we are 
using.

I have conversions that allow me to go back and forth between ode units and 
meters and everything works well.  I can make a vehicle wheel with a radius 
read from a spec sheet, then create a geom and body of the correct size to 
simulate it in my world.

My problem now, is figuring out vehicle speed in real world units. (m/s for 
instance).  I need to be able to relate my vehicle wheel's angular speed to 
my vehicles speed so that if I want my vehicle to go 50 m/s I know what I 
need to set for my wheel joint's dParamVel ("Desired Velocity")

I think my confusion is with the "time" unit. It's a pretty easy calculation 
to go from angular velocity to linear velocity (ignoring slip) if you know 
the radius of the wheel.  However, the value returned from 
GetHinge2Angle2Rate() does not jive with the real world.  The vehicles 
linear speed calculated from the angular speed returned from this function 
is wrong.

I'm sure it is because I assumed "seconds" as a unit of time.  My question 
is how can I figure out a unit of time that will give my correct results?  
Do I need to physically measure it within the context of my world?  For 
instance, knowing the radius of my wheel, and setting the "Desired Velocity" 
of my wheel to 1 should I measure how long it takes to go a certain distance 
and figure the time unit conversion from their?

Is there a better/easier way?

I apologize for the long post, but this is important for our project.

-jeff

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